1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a light-emitting element which includes a light-emitting layer containing an organic compound between a pair of electrodes. Furthermore, the present invention relates to a light-emitting device including the light-emitting element and an electronic device including the light-emitting device.
2. Description of the Related Art
In recent years, mobile devices show remarkable development, and one can easily enjoy image works anytime and anywhere with a small image reproducing device, a display of a smartphone or a tablet terminal, and the like. In addition, image data is being more often downloaded or transferred by using a small memory, and thus, the demand for the mobile devices is being increased.
In order to enjoy high-quality images with a small display such as a display of a mobile device, the display is required to have sufficiently high definition.
A light-emitting element (also referred to as an electroluminescence element or an EL element) which includes a light-emitting layer containing an organic compound between a pair of electrodes is capable of high-speed response and DC drive at low voltage, and can be manufactured to be thin and lightweight. Therefore, the light-emitting element is being put into practical use as a flat panel display element or a mobile display element.
An EL element includes a pair of electrodes and an EL layer containing a light-emitting substance, which is provided between the electrodes, and emits light when the light-emitting substance contained in the EL layer is excited by current flowing through the EL layer. Therefore, in order to obtain high emission intensity in such an EL element, current corresponding to the intensity needs to flow through the light-emitting layer, and power consumption is increased accordingly. In addition, as large current flows, degradation of the EL element is accelerated.
In view of the above, a light-emitting element which includes a stack of a plurality of EL layers and thereby capable of emitting light with higher luminance than a light-emitting element including only one EL layer, when current having the same current density flows through each light-emitting element, is proposed (e.g., see Patent Document 1). In a light-emitting element disclosed in Patent Document 1, a plurality of light-emitting units is separated from each other by a charge generation layer.